I also have no idea on photo sizing for Hive. I know that when you upload them, if they are too big they fail or they seem to upload, but don't display.
Your tracking suggestions sound good. It's something that's come up on a couple of occasions to know if images are being used. Currently we're encouraging the use of the tag #stockshares by those using the images in an attempt to find out how much they're being used. As you can guess, it's not very effective.
A site where images can be stored and searched would be great too. It's not within my expertise, though. I never really planned to make it more than a place to share images for use, but due to the interest it's evolved.
Happy for you to contact me on discord, if you want to chat further.